This invention relates to methods of, and apparatus for, surveying a borehole.
A spatial survey of the path of a borehole is usually derived from a series of values of the azimuth angle and the inclination angle taken along the length of the borehole. Measurements from which the values of these two angles can be derived are made at successive locations along the path of the borehole, the distances between adjacent locations being accurately known.
In a borehole in which the earth's magnetic field is unchanged by the presence of the borehole itself, measurements of the components of the earth's gravitational and magnetic fields in the direction of the casing-fixed axes can be used to obtain values for the azimuth angle and the inclination angle, the azimuth angle being measured with respect to an earth-fixed magnetic reference, for example magnetic North. However, in situations in which the earth's magnetic field is modified by the local conditions in a borehole, for example when the borehole is cased with a steel lining, magnetic measurements can no longer be used to determine the azimuth angle relative to an earth-fixed reference. In these circumstances, it is necessary to use a gyroscopic instrument.
Several gyroscopic instruments have been developed for this purpose and these have operated satisfactorily at inclination angles below a certain value. However, at inclination angles in excess of 60.degree. to the vertical, increasingly less accurate surveys result as the inclination increases. The present invention provides an entirely new surveying technique which is capable of producing very accurate surveys at any inclination angle and which is particularly applicable to the use of gyroscopic units having no moving parts which are of high accuracy and reliability.